A variety of applications exist for LCDs including flat panels, televisions, computers, and cameras. The development of a large, viable flat panel display technology and other related display technologies has been inhibited by the scarcity of suitable materials that exhibit optical transparency and electrical conductivity. A matrix LCD typically includes overlapping row and column electrodes that roughly define display elements that may respond to an applied bias. The row and column electrodes are generally positioned on opposite sides of a layer of liquid crystal material. Where row and column electrodes overlap, display elements are formed. The electro-optic material which forms the liquid crystal layer generally responds to a change in the value of an RMS voltage applied across the display element to provide a corresponding change in the amount of transmitted light. In popular passive displays, use of a super-twisted neumatic (STN) type of liquid crystal as an electro-optic material is common. Traditionally, thin layers of metal or metal oxide have been etched on an optically transparent substrate such as glass, for example, to form substantially transparent electrode structures. However, such designs and materials suffer from numerous shortcomings in optical and electrical characteristics. For example, commonly employed optically transparent metal oxide layers suffer from relatively high electrical sheet resistivity, limited current handling capacity and brittleness. Furthermore, manufacturing such oxide layers is relatively complex, requiring the use of elaborate fabrication techniques including laser etching and chemical vapor deposition resulting in a high cost of production. Similarly, in the case of conductive metal layers, optical transmissive loss has been inherent to known prior art techniques. Increased transmittance has been generally achieved by use of relatively thinner metal or metal oxide films or layers. Such thin films or layers, however, possess increased electrical sheet resistance and decreased current handling capacity, resulting in slower switching response in display device applications. Moreover, metal layers may exhibit pernicious reactions with electro-optic materials during operation of the display device leading to shortened lifetimes;
Display element capacitance is another significant parameter of the LCD electrode structure. The display element capacitance is a function of many variables including spacing between the crossover or overlap of the row and column electrodes, the properties of the dielectric material between the electrodes and the area of the electrodes at their intersection. The larger the size of the display element formed by the overlap area at the electrode intersection the bigger the display element capacitance since it is directly proportional to the overlap area. For the present description, with respect to the electrodes, “overlap” refers to being oriented in a manner that they are superimposed with the liquid crystal display material disposed between them.
The amount of time required for an electrode to reach an applied voltage is a function of the electrode resistance and the capacitance. The larger the capacitance and/or the resistance, the more time is required for the electrode and, consequently, the addressed display element to reach the applied voltage which corresponds to the data. High sheet resistivity of optically transparent and electrically conductive electrodes, capacitance associated with the display element, and significant loss of light through the electrode structures utilized in present passive matrix displays all make it difficult to manufacture large display area display panels.
Recently, numerous approaches and a variety of electrode structures using various metals and semiconductor oxides have been attempted. For example, U.S. Pat. No. 5,852,486 to Hoke purportedly uses vertical barrier members within the liquid crystal display rather than electrodes resident on substrates. Although electrode structures used by Hoke purportedly have increased stability and improved visual appearance, the Hoke electrode structure employs complex manufacturing process. The LCD material is to be encapsulated between substrates and vertical barrier members.
U.S. Pat. No. 5,556,530 to Finkelstein et al. discloses an array of electrodes purportedly employing electron emitters for use in large flat panel displays. However, inherent emitter design problems associated with emission electrodes potentially limits acceptance. U.S. Pat. No. 5,293,546 to Tadros et al. describes an electrode structure which purportedly uses a working electrode comprising a transparent metal grid with metal oxide coating in conjunction with a display device.
The novel electrode structure of the present invention mitigates several LCD performance limitations encountered in the electrode structures of the prior art.